(1) Field of the Invention
This invention relates to a displacement control valve, and more particularly to a displacement control valve for use in a variable displacement compressor for compressing a refrigerant gas in a refrigeration cycle for an automotive air conditioner.
(2) Description of the Related Art
A compressor used for compressing refrigerant in a refrigeration cycle for an automotive air conditioner is driven by an engine, and hence is not capable of controlling the rotational speed thereof. For this reason, a variable displacement compressor capable of changing the compression displacement for compressing refrigerant is employed so as to obtain adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In the above-mentioned variable displacement compressor, compression pistons are connected to a wobble plate fitted on a shaft driven rotatably by the engine, and the angle of the wobble plate is changed to change the length of piston stroke for changing the discharge amount of the compressor.
The angle of the wobble plate is continuously changed by introducing part of the compressed refrigerant into a gastight pressure-regulating chamber and changing the pressure of the introduced refrigerant, thereby changing a balance between pressures applied to the opposite ends of each piston.
A compression displacement control device disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-132650 has a solenoid control valve arranged between a discharge port and a pressure-regulating chamber of a compressor or between the discharge port and a suction port of the same. This solenoid control valve opens and closes the communication such that a differential pressure across the solenoid control valve is maintained at a predetermined value. The predetermined value of the differential pressure can be set from outside by a current value. As a result, when the engine rotational speed increases, the pressure introduced into the pressure-regulating chamber is increased to shorten piston stroke to thereby reduce the displacement for compression, while when the engine rotational speed decreases, the pressure introduced into the pressure-regulating chamber is reduced to lengthen the piston stroke to thereby increase the displacement for compression, whereby the pressure of refrigerant discharged from the compressor is maintained at a constant level.
Although refrigerant generally used in a refrigeration cycle of an automotive air conditioner is a chlorofluorocarbon alternative HFC-134a, there has recently been developed a refrigeration cycle which causes the refrigerant to perform refrigeration in a supercritical region where the temperature of the refrigerant is above its critical temperature, e.g. a refrigeration cycle using carbon dioxide as refrigerant
In the conventional solenoid control valve for the compression displacement control device, to maximize operating displacement of the variable displacement compressor, it is required to maximize the amount of refrigerant conducted out from the pressure-regulating chamber into the suction chamber to reduce pressure within the pressure-regulating chamber, but if the size of the valve is small, the amount of refrigerant conducted out is small, and hence transition to the maximum displacement operation takes time, which can degrade controllability of the compressor.
On the other hand, if the size of the valve is increased so as to increase the amount of refrigerant conducted out, the pressure-receiving area of the valve is also increased, and hence a large solenoid force is required to control the valve. Particularly in the refrigeration cycle using carbon dioxide as the refrigerant, since the pressure of refrigerant is increased to the supercritical region, the discharge pressure of the refrigerant becomes very high, so that the solenoid force for controlling the valve also becomes very large. This necessitates a huge solenoid, which causes an increase in the size of the solenoid valve and a resultant increase in manufacturing costs.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a displacement control valve which is capable of performing transition between operating displacements in a reduced time period and operating without using a large solenoid force even when the size of the valve is increased so as to increase the amount of refrigerant.
In order to accomplish the object, a displacement control valve for controlling an amount of refrigerant conducted out from a pressure-regulating chamber into a suction chamber, such that a differential pressure between pressure in the suction chamber and pressure in a discharge chamber is held at a predetermined differential pressure, to thereby change an amount of the refrigerant discharged from a variable displacement compressor is provided. The displacement control valve is characterized by comprising the steps of; (a) a valve section for opening and closing a refrigerant passage between the pressure-regulating chamber and the suction chamber to control the amount of refrigerant conducted out from the pressure-regulating chamber to the suction chamber; (b) a differential pressure-sensing section that is formed separately from the valve section, for sensing the differential pressure between the pressure in the discharge chamber and the pressure in the suction chamber, thereby controlling a valve travel of the valve section; and (c) a solenoid section for having a current value supplied thereto changed to change a solenoid force thereof applied to a valve element of the valve section to thereby change the predetermined differential pressure so as to control a discharge amount of the refrigerant.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.